This invention relates to porous films, in particular porous films having a substantially regular structure and uniform pore size, and to a method of preparing porous films by electrodeposition.
Porous films and membranes have found extensive applications as electrodes and solid electrolytes in electrochemical devices and sensors. Their open and interconnected microstructure maximises the area over which interaction and/or redox processes can occur, allows electrical conduction, and minimises distances over which mass transport has to occur in order to ensure efficient device operation.
Conventional processes for preparing porous films include the sintering of small particles, deposition from vapour phase reactants, chemical etching and electrodeposition from multicomponent plating solutions. These processes tend to produce materials with a variable pore size, generally in the macroporous range, and with variable thickness of the walls separating the pores. Consequently, these materials may not have sufficiently large specific surface areas, and their irregular structure does not allow for optimum mass transport or electrical conductivity, and may result in poor mechanical and chemical stability.
In the drive towards providing porous films showing improved properties, for use in for example batteries, fuel cells, electrochemical capacitors, light-to-electricity conversion, quantum confinement effect devices, sensors, magnetic devices, superconductors, electrosynthesis and electrocatalysis, to our knowledge no one has yet succeeded in developing an effective process for preparing at least mesoporous films of regular structure and uniform pore size, with the attendant advantages in terms of properties which such films might be expected to show.
For example, previous reported attempts to form polypyrrole films by electrodeposition, from thermotropic liquid crystalline phases, resulted in films of only weakly anisotropic structure.
Previously, we have shown that porous, non-film, materials such as ceramic oxide monoliths and metal powders can be crystallised, gelled or precipitated from lyotropic liquid crystalline phase media, whereby the liquid crystalline phase topology directs the synthesis of the material into a corresponding topology showing structural regularity and uniformity of pore size. However, it was not expected that this templating mechanism could be used to synthesise porous materials other than by simple crystallisation, gelation or precipitation.
What we have found, surprisingly, is that porous films can be prepared from an homogeneous lyotropic liquid crystalline phase by electrodeposition. Surfactants have previously been used as additives in electroplating mixtures in order to enhance the smoothness of electrodeposited films or to prevent hydrogen sheathing (see for example J. Yahalom, O. Zadok, J. Materials Science (1987), vol 22, 499-503). However, in all cases the surfactant was used at concentrations that are much lower than those required to form liquid crystalline phases. Indeed, in these applications high surfactant concentrations were hitherto regarded as undesirable because of the increased viscosities of the plating mixtures.
The present invention in a first aspect provides a method of preparing a porous film which comprises electrodepositing material from a mixture onto a substrate to form a porous film, wherein the mixture comprises:
a source of metal, inorganic oxide, non-oxide semiconductor/conductor or organic polymer, or a combination thereof;
a solvent; and
a structure-directing agent in an amount sufficient to form an homogeneous lyotropic liquid crystalline phase in the mixture, and optionally removing the organic directing agent.
In a second aspect, the invention provides a porous film electrodeposited onto a substrate, wherein the film has a regular structure such that recognisable architecture or topological order is present in the spatial arrangement of the pores in the film, and a uniform pore size such that at least 75% of the pores have pore diameters to within 40% of the average pore diameter.
According to the method of the invention, an homogeneous lyotropic liquid crystalline mixture is formed for electrodeposition onto a substrate. The deposition mixture comprises a source material for the film, dissolved in a solvent, and a sufficient amount of an organic structure-directing agent to provide an homogeneous lyotropic liquid crystalline phase for the mixture. A buffer may be included in the mixture to control the pH.
Any suitable source material capable of depositing the desired species onto the substrate by electrodeposition may be used. By xe2x80x9cspeciesxe2x80x9d in this context is meant metal, inorganic oxide, including metal oxide, non-oxide semiconductor/conductor or organic polymer. Suitable source materials will be apparent to the person skilled in the art by reference to conventional electroplating or electrodeposition mixtures.
One or more source materials may be included in the mixture in order to deposit one or more species. Different species may be deposited simultaneously from the same mixture. Alternatively, different species may be deposited sequentially into layers from the same mixture, by varying the potential such that one or another species is preferentially deposited according to the potential selected.
Similarly, one or more source materials may be used in the mixture in order to deposit one or more materials selected from a particular species or combination of species, either simultaneously or sequentially. Thus, by appropriate selection of source material and electrodeposition regime, the composition of the deposited film can be controlled as desired.
Suitable metals include for example Group IIB, IIIA-VIA metals, in particular zinc, cadmium, aluminium, gallium, indium, thallium, tin, lead, antimony and bismuth, preferably indium, tin and lead; first, second and third row transition metals, in particular platinum, palladium, gold, rhodium, ruthenium, silver, nickel, cobalt, copper, iron, chromium and manganese, preferably platinum, palladium, gold, nickel, cobalt, copper and chromium, and most preferably platinum, palladium, nickel and cobalt; and lanthanide or actinide metals, for example praseodymium, samarium, gadolinium and uranium.
The metals may contain surface layers of, for example, oxides, sulphides or phosphides.
The metals may be deposited from their salts as single metals or as alloys.
Thus, the film may have a uniform alloy composition, for example Ni/Co, Ag/Cd, Sn/Cu, Sn/Ni, Pb/Mn, Ni/Fe or Sn/Li, or if deposited sequentially, a layered alloy structure, for example Co/Cu|Cu/Co, Fe/Co|Co/Fe or Fe/Cr|Cr/Fe, wherein xe2x80x9cCo/Cu|Cu/Coxe2x80x9d denotes a film containing alternate layers of cobalt-rich alloy and copper-rich alloy. Sequential electrodeposition of species can be achieved according to the method disclosed by Schwarzacher et al., Journal of Magnetism and Magnetic Materials (1997) vol 165, p23-39. For example, an hexagonal phase is prepared from an aqueous solution containing two metal salts A and B, where metal A is more noble than metal B (for example nickel (II) sulphate and copper (II) sulphate) and optionally a buffer (for example boric acid). The deposition potential is alternated from a value only sufficiently negative to reduce A, to one that is sufficiently negative to reduce both A and B. This gives and produces an alternating layered structure consisting of layers A alternating with layers A+B.
Suitable oxides include oxides of for example first, second and third row transition metals, lanthanides, actinides, Group IIB metals, Group IIIA-VIA elements, preferably oxides of titanium, vanadium, tungsten, manganese, nickel, lead and tin, in particular titanium dioxide, vanadium dioxide, vanadium pentoxide, manganese dioxide, lead dioxide and tin oxide.
In some cases, the oxides may contain a proportion of the hydrated oxide i.e. contain hydroxyl groups.
The oxides may be deposited either as single oxides or as mixed oxides, and may optionally be deposited together with a Group IA or Group IIA metal to provide a doped oxide film.
Suitable non-oxide semiconductors/conductors include elemental types such as germanium, silicon and selenium, binary types such as gallium arsenide, indium stibnate, indium phosphide and cadmium sulphide, and other types such as Prussian Blue and analogous metal hexacyanometallates. Electrodeposition of semiconductors can be achieved using the source materials disclosed by: S. K. Das, G. C. Morris, J. Applied Physics (1993), vol 73, 782-786; M. P. R. Panicker, M. Knaster, F. A. Kroger, J. Electrochem. Soc. (1978), vol 125, 566-572; D. Lincot et at., Applied Phys. Letters (1995), vol 67, 2355-2357; M. Cocivera, A Darkowski, B. Love, J. Electrochem. Soc. (1984), vol 131, 2514-2517. J.-F. Guillemoles et al., J. Applied Physics (1996), vol 79, 7293-7302; S. Cattarin, F. Furlanetto, M. M. Musiani, J. Electroanalyt. Chem (1996), vol 415, 123-132; R. Dorin, E. J. Frazer, J. Applied Electrochem. (1998), vol 18, 134-141; M.-C. Yang, U. Landau, J. C. Angus, J. Electrochem. Soc. (1992), vol 139, 3480-3488.
Suitable organic polymers include aromatic and olefinic polymers, for example conducting polymers such as polyaniline, polypyrrole and thiophene, or derivatives thereof These will generally be associated with organic or inorganic counterions, for example chloride, bromide, sulphate, sulphonate, tetrafluoroborate, hexafluorophosphate, phosphate, phosphonate, or combinations thereof.
Other suitable organic materials include insulating polymers such as polyphenol, polyacrylonitrile and poly(ortho-phenylene diamine).
One or more solvents are included in the mixture in order to dissolve the source material and to form a liquid crystalline phase in conjunction with the structure-directing agent, thereby to provide a medium from which the film may be electrodeposited. Generally, water will be used as the preferred solvent. However, in certain cases it may be desirable or necessary to carry out the electrodeposition in a non-aqueous environment. In these circumstances a suitable organic solvent may be used, for example formamide, ethylene glycol or glycerol.
One or more structure-directing agents are included in the mixture in order to impart an homogeneous lyotropic liquid crystalline phase to the mixture. The liquid crystalline phase is thought to function as a structure-directing medium or template for film deposition. By controlling the nanostructure of the lyotropic liquid crystalline phase, and electrodepositing, a film may be synthesised having a corresponding nanostructure. For example, films deposited from normal topology hexagonal phases will have a system of pores disposed on an hexagonal lattice, whereas films deposited from normal topology cubic phases will have a system of pores disposed in cubic topology. Similarly, films having a lamellar nanostructure may be deposited from lamellar phases.
Accordingly, by exploiting the rich lyotropic polymorphism exhibited by liquid crystalline phases, the method of the invention allows precise control over the structure of the films and enables the synthesis of well-defined porous films having a long range spatially and orientationally periodic distribution of uniformly sized pores.
Any suitable amphiphilic organic compound or compounds capable of forming an homogeneous lyotropic liquid crystalline phase may be used as structure-directing agent, either low molar mass or polymeric. These compounds are also sometimes referred to as organic directing agents. In order to provide the necessary homogeneous liquid crystalline phase, the amphiphilic compound will generally be used at an high concentration, typically at least about 10% by weight, preferably at least 20% by weight, and more preferably at least 30% by weight, based on the total weight of the solvent and amphiphilic compound.
Suitable compounds include organic surfactant compounds of the formula RQ wherein R represents a linear or branched alkyl, aryl, aralkyl or alkylaryl group having from 6 to about 6000 carbon atoms, preferably from 6 to about 60 carbon atoms, more preferably from 12 to 18 carbon atoms, and Q represents a group selected from: [O(CH2)m]nOH wherein m is an integer from 1 to about 4 and preferably m is 2, and n is an integer from 2 to about 100, preferably from 2 to about 60, and more preferably from 4 to 8; nitrogen bonded to at least one group selected from alkyl having at least 4 carbon atoms, aryl, aralkyl and alkylaryl; and phosphorus or sulphur bonded to at least 2 oxygen atoms. Preferred examples include cetyl trimethylammonium bromide, sodium dodecyl sulphate, sodium dodecyl sulphonate and sodium bis(2-ethylhexyl) sulphosuccinate.
Other suitable structure-directing agents include monoglycerides, phospholipids, glycolipids and amphiphilic block copolymers.
Preferably non-ionic surfactants such as octaethylene glycol monododecyl ether (C12EO8, wherein EO represents ethylene oxide), octaethylene glycol monohexadecyl ether (C16EO8) and non-ionic surfactants of the Brij series (trade mark of ICI Americas), are used as structure-directing agents.
In most cases, the source material will dissolve in the solvent domains of the liquid crystalline phase, but in certain cases the source material may be such that it will dissolve in the hydrophobic domains of the phase.
The mixture may optionally further include a hydrophobic additive to modify the structure of the phase, as explained more fully below. Suitable additives include n-heptane, n-tetradecane, mesitylene and triethyleneglycol dimethyl ether. The additive may be present in the mixture in a molar ratio to the structure-directing agent in the range of 0.1 to 10, preferably 0.5 to 2, and more preferably 0.5 to 1.
The mixture may optionally further include an additive that acts as a co-surfactant, for the purpose of modifying the structure of the liquid crystalline phase or to participate in the electrochemical reactions. Suitable additives include n-dodecanol, n-dodecanethiol and perfluorodecanol. The additive may be present in the mixture in a molar ratio to the structure-directing agent in the range of 0.01 to 2, and preferably 0.08 to 1.
The deposition mixture is electrodeposited onto a suitable substrate, for example a polished gold, copper or carbon electrode. The specific electrodeposition conditions of pH, temperature, potential, current density and deposition period will depend on the source material used and the thickness of film to be deposited. Typically, the pH of the deposition mixture is adjusted to a value in the range from I to 14, and preferably in the range from 2 to 6 or from 8 to 12. The current density for galvanostatic deposition is generally in the range from 1 pA/cm2 to 1 A/cm2. Typically, for potentiostatic deposition at fixed potential, the potential applied has a value in the range xe2x88x9210V to +10V, preferably xe2x88x923V to +3V, and more preferably xe2x88x921V to +1V, relative to the standard calomel electrode. Typically, for potentiostatic deposition at variable potential, the applied potential is stepped between fixed limits generally within the range from xe2x88x9210V to +10V, relative to the standard calomel electrode, or swept at a rate in the range from 1 mV/s to 100 kV/s. The temperature is generally in the range from 15 to 80xc2x0 C., preferably 20 to 40xc2x0 C. The electrodeposition will generally be carried out so as to deposit a film of a thickness from 1 nm (10 xc3x85) to 200 xcexcm, preferably 2 nm (20 xc3x85) to 100xcexcm, more preferably 5 nm (50 xc3x85) to 50 xcexcm, and still more preferably 10 nm (100 xc3x85) to 20xcexcm.
It will be appreciated that the conditions under which electrodeposition is conducted may be varied so as to control the nanostructure and properties of the deposited film. For example, we have found that the temperature at which electrodeposition is conducted affects the double layer capacitance of the films. Also, the deposition potential affects the regularity of the nanostructure.
Following electrodeposition, it will usually be desirable to treat the film to remove the structure-directing agent, any hydrocarbon additive and co-surfactant, unreacted source material and ionic impurities, for example by solvent extraction or by decomposition in nitrogen and combustion in oxygen (calcination). However, for certain applications such treatment may not be necessary.
The deposited film may then optionally be subjected to further treatment, for example to the electrochemical or chemical insertion of ionic species, to the physical absorption of organic, inorganic or organometallic species, to electrodeposition, solution phase deposition or gas phase deposition of organic, inorganic or organometallic species onto the internal surfaces so as to create thin coatings, or onto the topmost surface, or into the pores so as to fill them partially or completely, to chemical treatment to form surface layers, for example by reaction with hydrogen sulphide gas to form metal sulphide or by adsorption of alkane thiols or other surface active materials, to physical treatment, for example by adsorption of proteins such as enzymes, by deposition of lipid bilayer overlayers as supports for transmembrane or membrane-associated proteins or by doping with Group I or II metals, or to thermal treatment, for example to form nanostructured carbon from electrodeposited polyphenol or polyacrylonitrile films.
It will be appreciated that the film may be used in situ as deposited on the substrate, or may be separated from the substrate after its deposition, according to its intended field of application. If separated, any optional post-deposition treatment of the film may be effected before, during or after separation of the film from the substrate.
It has been found that the pore size of the deposited film can be varied by altering the hydrocarbon chain length of the surfactant used as structure-directing agent, or by supplementing the surfactant by an hydrocarbon additive. For example, shorter-chain surfactants will tend to direct the formation of smaller-sized pores whereas longer-chain surfactants tend to give rise to larger-sized pores. The addition of an hydrophobic hydrocarbon additive such as n-heptane, to supplement the surfactant used as structure-directing agent, will tend to increase the pore size, relative to the pore size achieved by that surfactant in the absence of the additive. Also, the hydrocarbon additive may be used to alter the phase structure of the liquid crystalline phase in order to control the corresponding regular structure of the deposited film.
Using the method according to the invention, regular porous films that are conducting or semiconducting phases can be prepared with pore sizes in mesoporous and macroporous ranges, possibly up to a pore size of about 30 nm (300 xc3x85). By xe2x80x9cmesoporousxe2x80x9d as referred to herein is meant a pore diameter within the range from about 1.3 to 20 nm (13 to 200 xc3x85), and by xe2x80x9cmacroporousxe2x80x9d is meant pore diameters exceeding about 20 nm (200 xc3x85). Preferably, the films are mesoporous, more preferably having a pore diameter within the range from 1.4 to 10 nm (14 to 100 xc3x85), and most preferably within the range from 1.7 to 4 nm (17 to 40 xc3x85).
The films in accordance with the invention may exhibit pore number densities in the range from 1xc3x971010 to 1xc3x971014 pores per cm2, preferably from 4xc3x971011 to 3xc3x971013 pores per cm2, and more preferably from 1xc3x971012 to 1xc3x971013 pores per cm2.
The porous film has pores of substantially uniform size. By xe2x80x9csubstantially uniformxe2x80x9d is meant that at least 75% of pores have pore diameters to within 40%, preferably within 30%, more preferably within 10%, and most preferably within 5%, of average pore diameter.
The film in accordance with the invention is of a substantially regular structure. By xe2x80x9csubstantially regularxe2x80x9d as used herein is meant that a recognisable topological pore arrangement is present in the film. Accordingly, this term is not restricted to ideal mathematical topologies, but may include distortions or other modifications of these topologies, provided recognisable architecture or topological order is present in the spatial arrangement of the pores in the film. The regular structure of the film may for example be cubic, lamellar, oblique, centred rectangular, body-centred orthorhombic, body-centred tetragonal, rhombohedral, hexagonal, or distorted modifications of these. Preferably the regular structure is hexagonal.